Emissions regulations for internal combustion engines have become more stringent over recent years. The regulated emissions of NOx and particulates from internal combustion engines are low enough that in many cases the emissions levels cannot be met with improved combustion technologies. Therefore, the use of exhaust aftertreatment systems on engines to reduce emissions is increasing.
Many conventional exhaust aftertreatment systems provide adequate emissions reduction during steady state operating conditions of the internal combustion engine. However, some conventional exhaust aftertreatment systems are ill-equipped to handle emissions generated during transient operations of the engine. As an engine transitions from a steady state operating condition to a transient operating condition, the emissions generated by the engine can change in an often dramatic manner. Particulate matter emissions spikes can result in faster loading of particulate filters in the exhaust aftertreatment system, which requires longer and more frequent regeneration of the filters and can cause more wear on and a reduced useful life of the filters. Additionally, NOx emissions spikes can result in a lower NOx reduction efficiency and higher NOx amounts emitted from the tailpipe.
Generally, exhaust aftertreatment systems attempt to compensate for changes in exhaust emissions, e.g., NOx and particulate emissions spikes, by altering various properties of the engine and/or components of the aftertreatment system to meet desired targets. For example, the temperature of the exhaust can be changed by introducing or reducing fuel into the exhaust or increasing the EGR fraction. Similarly, the amount of emissions reduced in the exhaust aftertreatment system can be changed by increasing the temperature of various catalysts or altering the amount of reductant injected into the exhaust stream if the aftertreatment system includes a selective catalytic reduction (SCR) catalyst. Also, some engine systems include air and fuel controllers that increase or decrease the air-to-fuel ratio introduced into the cylinder of an engine in response to transient operating conditions.
Typically, a controller will generate a reference target command corresponding to the desired target. However, the desired changes or transient calibrations in the engine and/or aftertreatment system properties necessary to compensate for the transient emissions changes are often slowly achieved. In other words, many of the desired changes often require a slow ramping-up process before the changes are reached (see, e.g., the open loop response 310 of FIG. 4). For example, a desired amount of oxygen in the cylinder corresponding to a reduction in harmful exhaust emissions may only be achieved a significant time after the desired amount of oxygen is requested by a systems controller. Because of the delay between the reference target command and achievement of the desired target, compensation for the transient exhaust emissions spikes is also delayed. Such transient exhaust emissions spikes can result in undesirable consequences, such as, for example, low control system model fidelity, inefficient emissions reduction, wear on the aftertreatment system, and damage to the aftertreatment system.
However, if the particulate matter and NOx emissions spikes during transient operations of the vehicle are regulated, e.g., reduced, the desired changes and transient calibrations can be achieved more quickly, which would result in exhaust aftertreatment systems more readily compensating for emissions spikes and an increase in emissions reduction efficiency. Moreover, lower emission spikes would result in lower particulate matter build-up rates on particulate matter filters, which would increase the life and durability of the filters.